Fiber lasers providing short laser pulses (pulse duration shorter than 1 ps, also referred to as ultrashort pulses or femtosecond pulses) and high pulse energy (of typically more than 1 nJ), good beam quality and excellent optical characteristics have applications in many fields of scientific research and industry. There has been great progress in developing short pulse fiber lasers.
However, difficulties remain associated with short laser pulses propagating in optical fibers over longer distances. Nonlinearities causing distortions in the spectrum as well as dispersion render the delivery of short laser pulses over distances of several meters (as often required in practical applications) complicated.
U.S. Pat. No. 5,862,287 describes an apparatus and method for delivery of dispersion compensated ultrashort optical pulses with high peak power. The known apparatus comprises a pulsed laser source which produces ultrashort optical pulses having a high peak power. Prior to transmitting the optical pulses through an optical delivery fiber, the temporal pulse width of the optical pulses is stretched, forming chirped optical pulses having a lower peak power. The pulse stretching may be performed within the laser or by a separate dispersive element (stretcher). The stretched optical pulses are transmitted through an optical fiber which delivers the pulses over a distance of several meters to an optical device where the laser pulses are used in a respective application. Because the peak power of the optical pulses is reduced by the stretching of the temporal pulse width nonlinear effects are reduced for the most part. The optical delivery fiber introduces a dispersion which compensates for the dispersion introduced by the pulsed laser source and the stretcher, such that a recompressed optical pulse is delivered to the application. Ideally, the optical delivery fiber also compensates for the dispersion introduced by optical components in the optical device used in the respective application, so that the laser pulses are fully recompressed at a point of interest, such as, for example, at a specimen or at a detector.
However, even with the known approach of stretching and recompressing the laser pulses the fiber delivery of laser pulses in the 1 nJ pulse energy-region with sub-100 fs pulse duration often results in a poor pulse quality. Typically, less than 60 percent of the total pulse energy is contained in the main peak of the laser pulse. Satellite pulses occur at the end of the delivery optical fiber wherein the energy contained in the satellite pulses is generally not usable. The satellite pulses might even saturate a nonlinear medium used in the respective application, as it might be the case for the generation of THz radiation.
In a typical known system (see F. Eichhorn at al., Opt. Express, vol. 18, no. 7, p. 6978, 2010), the laser pulses generated by a seed laser source are amplified and solitonically compressed in an anomalous dispersion fiber. Before temporal stretching in a dispersion compensating fiber (DCF) the laser pulses have their minimum pulse duration. Finally, the laser pulses are compressed in a standard optical fiber (SMF-28 or PM1550) used as delivery fiber. This method has major drawbacks. Due to the very short pulse duration third order dispersion becomes important. This can be compensated to a certain extent by choosing a carefully balanced third order dispersion of the DCF. Furthermore, the laser pulses obtain a strongly modulated spectral shape by solitonic compression. When stretching the laser pulse in the DCF they acquire a temporal shape that is similar to their spectral shape (analogous to Fraunhofer diffraction in the time-domain). During the following compression self-phase modulation (SPM) proportional to the temporal shape of the laser pulses causes a complex phase profile that prevents an efficient recompression. This results in a strongly structured pulse shape at the end of the delivery fiber with a significant part of the pulse energy being contained in the satellites.